The present invention relates to communication apparatus, and more particularly to a homodyne receiver for use in a cable television converter or the like.
Homodyne receivers are well known for use in radar applications. Such receivers multiply ("mix") a desired signal by a carrier of the same frequency as the signal. A tunable oscillator, such as a voltage controlled oscillator ("VCO") is used to provide the local oscillator signal providing the mixing frequency. This frequency is mixed with the input signal to provide an in-phase output "I". The local oscillator frequency is also phase shifted by 90.degree. and mixed with the input signal to provide a quadrature output signal "Q". The I and Q signals are processed to recover the desired baseband signal.
Homodyne receivers have not been considered appropriate for use in cable television and similar applications for various reasons. In a homodyne receiver, the frequency spectrum folds over itself when the desired signal is multiplied by a carrier of the same frequency. This result is unacceptable in a television environment, since interference with adjacent television channels would result. Interference also arises within the desired channel.
A standard homodyne receiver also requires that the 90.degree. phase shift between the I and Q signals be tracked throughout the signal recovery process. This is difficult and expensive to do in a cable television converter.
The cable television band currently spans a frequency range of 50-550 MHz. This range is expected to expand to 50-1000 MHz in the future. A homodyne receiver for use in such a frequency range would have to include a tunable local oscillator and phase shifter operable over the entire cable television band. It would be prohibitively costly to provide a tunable oscillator having such an extended range, where the ratio between the highest and lowest frequencies is greater than 10.
Further, it is much more difficult to control phase noise in a homodyne receiver than in a conventional television receiver. Any spurious signal in the demodulated signal must be -60 dBc, since signals of the same frequency (or within the baseband bandwidth limits) will create spurious products in the desired signal. In order to solve this problem, the harmonics of the tunable local oscillator in a homodyne receiver would have to be 60 dB down. Alternately, switchable filtering would have to be used at the output of the receiver. Neither option is economically feasible.
Use of a homodyne receiver in a cable television or similar application is further complicated in that a high degree of isolation would be required from the mixers and the input amplifier since the local oscillator is at the frequency of the desired channel. The requirement for high quality components renders the use of a conventional homodyne receiver too expensive for cable television applications.
One benefit of a homodyne receiver is that it provides I and Q outputs that are susceptible to digital processing. It is desirable to use digital signal processing in a cable television converter to eliminate adjustments and provide a more reliable, less expensive product. Ideally, the use of digital signal processing would enable a cable television converter/descrambler to be designed using a single VLSI integrated circuit chip.
It would be advantageous to provide a homodyne receiver for use in a cable television converter or the like that provides the benefit of digital signal processing, without the substantial disadvantages inherent in conventional homodyne receivers identified above. The present invention provides such a receiver.